Cell-free protein synthesis (CFPS) platforms have emerged as a powerful technology for protein expression. Prominent applications include the production of pharmaceutical proteins and vaccines (Goerke, A. R. et al. “Development of cell-free protein synthesis platforms for disulfide bonded proteins,” Biotechnol. Bioeng. 99, 351-367 (2008); Kanter, G. et al. “Cell-free production of scFv fusion proteins: An efficient approach for personalized lymphoma vaccines,” Blood 109, 3393-3399, (2007); Stech, M. et al. “Production of functional antibody fragments in a vesicle-based eukaryotic cell-free translation system,” J. Biotechnol. 164, 220-231 (2012); Yang, J. et al. “Rapid expression of vaccine proteins for B-cell lymphoma in a cell-free system,” Biotechnol. Bioeng. 89, 503-511 (2005); Yin, G. et al. “Aglycosylated antibodies and antibody fragments produced in a scalable in vitro transcription-translation system,” MAbs 4, 217-225 (2012); Zawada, J. F. et al. “Microscale to manufacturing scale-up of cell-free cytokine production—a new approach for shortening protein production development timelines,” Biotechnol. Bioeng. 108, 1570-1578 (2011)). Such systems enable expression in vitro of proteins that are difficult to produce in vivo, as well as high-throughput production of protein libraries for protein evolution, functional genomics, and structural studies (Madin, K. et al. “A highly efficient and robust cell-free protein synthesis system prepared from wheat embryos: Plants apparently contain a suicide system directed at ribosomes,” Proc. Natl. Acad. Sci. U.S.A. 97, 559-564 (2000); Takai, K et al. “Practical cell-free protein synthesis system using purified wheat embryos,” Nat. Protoc. 5, 227-238 (2010)). Prokaryotic Escherichia coli extract based cell-free systems have developed rapidly (for a review, see Carlson, E. D. et al. “Cell-free protein synthesis: Applications come of age,” Biotechnol. Adv. 30, 1185-1194, (2012)). Yet an integrated eukaryotic platform with similar productivity, scalability, protein folding capability, and cost effectiveness has lagged behind.
The major eukaryotic CFPS platforms previously developed include systems made from wheat germ extract (WGE) (Goshima, N. et al. “Human protein factory for converting the transcriptome into an in vitro-expressed proteome,” Nat. Methods 5, 1011-1017 (2008); Hoffmann, M. et al. in Biotechnol Annu Rev Vol. 10, 1-30 (Elsevier, 2004); Takai et al. (2010)), rabbit reticulocyte lysate (RRL) (Jackson, R. J. et al. in Methods Enzymol Vol. Vol. 96 (eds. Becca Fleischer, Sidney Fleischer) Ch. 4, 50-74 (Academic Press, 1983)); insect cell extract (ICE) (Ezure, T et al. “A cell-free protein synthesis system from insect cells,” Methods Mol. Biol. 607, 31-42 (2010); Kubick, S et al. in Current Topics in Membranes, Vol. 63 (ed. Larry DeLucas) 25-49 (Academic Press, 2009); Tarui, H. et al. “Establishment and characterization of cell-free translation/glycosylation in insect cell (Spodoptera frugiperda 21) extract prepared with high pressure treatment,” Appl. Microbiol. Biotechnol. 55, 446-453 (2001)); Leishmania tarentolae extract (Kovtun, O. et al. “Towards the construction of expressed proteomes using a Leishmania tarentolae based cell-free expression system,” PLoS One 5, e14388 (2010); Mureev, S. et al. “Species-independent translational leaders facilitate cell-free expression,” Nat. Biotechnol. 27, 747-752 (2009)); and HeLa and hybridoma cell extract (Mikami, S. et al. in Cell-Free Protein Production Vol. 607 Methods in Molecular Biology (eds. Yaeta Endo, Kazuyuki Takai, & Takuya Ueda) Ch. 5, 43-52 (Humana Press, 2010)).
Compared to the E. coli system, these methods have advantages for producing some types of complex proteins and can achieve post-translational modifications not found in bacteria (Chang, H.-C. et al. “De novo folding of GFP fusion proteins: High efficiency in eukaryotes but not in bacteria,” J. Mol. Biol. 353, 397-409 (2005)). Insect cell-extract systems, for example, have demonstrated acetylation and N-myristoylation (Suzuki, T. et al. “N-terminal protein modifications in an insect cell-free protein synthesis system and their identification by mass spectrometry,” Proteomics 6, 4486-4495 (2006)); isoprenylation (Suzuki, T. et al. “Protein prenylation in an insect cell-free protein synthesis system and identification of products by mass spectrometry,” Proteomics 7, 1942-1950 (2007)); ubiquitination (Suzuki, T. et al. “Preparation of ubiquitin-conjugated proteins using an insect cell-free protein synthesis system,” J. Biotechnol. 145, 73-78 (2010)), core glycosylation (Merk, H. et al. “Cell-free synthesis of functional and endotoxin-free antibody Fab fragments by translocation into microsomes,” Biotechniques 53, 153-160 (2012); Tarui et al. (2001)); disulfide bond formation in single chain antibody fragments (Stech et al. (2012)); and significant advances in expression and modification of membrane bound proteins (Kubick et al. (2009)). However, eukaryotic cell-free platforms often have limited batch protein yields (Carlson et al. (2012)), or depend on costly and inefficient continuous exchange reactions that do not scale commercially (Zawada et al. (2011)). Furthermore, eukaryotic CFPS systems are generally limited by laborious and expensive extract preparation methods. For example, WGE, which is the most common eukaryotic system, requires lengthy preparation steps that include grinding, sieving, extensive washing, and eye selection of the embryo to ensure the embryo is in the proper stage of development (Takai et al. (2010)). An additional challenge of this approach is that approximately 5 mL of active extract is produced from 5 to 6 kg of starting material after 4 to 5 days of processing (Id.) In contrast, E. coli can be processed quickly and under precise growth conditions to develop a highly active and robust CFPS platform, where 60 g of cells (wet weight) can be converted to 120 mL of extract in only 4-6 hours of preparation (Liu, D. V. et al. “Streamlining Escherichia coli S30 extract preparation for economical cell-free protein synthesis,” Biotechnol Prog 21, 460-465 (2005)). The above limitations motivate the need for a new eukaryotic CFPS platform that is robust, easy to prepare, highly active, and amenable to economical scale-up.
S. cerevisiae, like E. coli, is a microbe that can be grown quickly under precise conditions in either a bioreactor or shake flasks. Furthermore, S. cerevisiae as a eukaryotic organism is suited to fold eukaryotic proteins and has previously shown some ability for post-translational modifications in vitro, such as glycosylation (Rothblatt, J. A. et al. “Secretion in yeast: Reconstitution of the translocation and glycosylation of alpha-factor and invertase in a homologous cell-free system,” Cell 44, 619-628 (1986)). Because it is a model organism for molecular study, S. cerevisiae is well understood at the biochemical level, has a wealth of documented “omics” that can prove useful when trying to characterize a cell-free system, and genetic tools are readily available for facile changes to the host strain (Nielsen, J. et al. “Impact of systems biology on metabolic engineering of Saccharomyces cerevisiae,” FEMS Yeast Res. 8, 122-131 (2008)). S. cerevisiae is also an important bio-manufacturing production platform and accounted for 18.5% of all FDA and EMA licensed recombinant protein biopharmaceuticals as of January 2009 (Ferrer-Miralles, N., et al. “Microbial factories for recombinant pharmaceuticals,” Microb. Cell. Fact. 8, 17 (2009)).
Despite these attractive features, yeast based CFPS systems have not been extensively developed as a protein synthesis platform since their origin in the 1970s and early 1980s (Gasior, E. et al. “The analysis of intermediary reactions involved in protein synthesis, in a cell-free extract of Saccharomyces cerevisiae that translates natural messenger ribonucleic acid,” J. Biol. Chem. 254, 3970-3976 (1979); Gasior, E. et al. “The preparation and characterization of a cell-free system from Saccharomyces cerevisiae that translates natural messenger ribonucleic acid,” J. Biol. Chem. 254, 3965-3969 (1979)). Instead, the majority of research involving yeast cell-free translation systems has focused on investigating translation from a fundamental perspective, such as elucidating cap-dependent translation (Iizuka, N. et al. “Cap-dependent and cap-independent translation by internal initiation of mRNAs in cell extracts prepared from Saccharomyces cerevisiae,” Mol. Cell. Biol. 14, 7322-7330 (1994); Iizuka, N. & Sarnow, P. “Translation-competent extracts from Saccharomyces cerevisiae: Effects of L-A RNA, 5′ cap, and 3′ poly(A) tail on translational efficiency of mRNAs,” Methods 11, 353-360 (1997)) and characterizing translation initiation factors (Algire, M. A. et al. “Development and characterization of a reconstituted yeast translation initiation system,” RNA 8, 382-397 (2002); Hinnebusch, A. G., et al. “Mechanism of translation initiation in the yeast Saccharomyces cerevisiae,” pp. 225-268 in Translational Control in Biology and Medicine, (eds. M. B. Mathews, N. Sonenberg and J. W. B. Hershey) (Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y. (2007); Kurata, S. et al. “Ribosome recycling step in yeast cytoplasmic protein synthesis is catalyzed by eEF3 and ATP,” Proc. Natl. Acad. Sci. U.S.A. 107, 10854-10859 (2010); Saini, P. et al. “Hypusine-containing protein eIF5A promotes translation elongation,” Nature 459, 118-121 (2009); Thompson, S. R. et al. “Internal initiation in Saccharomyces cerevisiae mediated by an initiator tRNA/eIF2-independent internal ribosome entry site element,” Proc. Natl. Acad. Sci. U.S.A 98, 12972-12977 (2001)). Despite this focus, some recent work has shown the potential to use yeast CFPS for making proteins of interest, such as virus-like particles (Wang, X. et al. “An optimized yeast cell-free system: Sufficient for translation of human papillomavirus 58 L1 mRNA and assembly of virus-like particles,” J. Biosci. Bioeng. 106, 8-15 (2008); Wang, X. et al. “Translational comparison of HPV58 long and short L1 mRNAs in yeast (Saccharomyces cerevisiae) cell-free system,” J. Biosci. Bioeng. 110, 58-65 (2010)) and additional viral proteins (Pogany, J. et al. “Authentic replication and recombination of tomato bushy stunt virus RNA in a cell-free extract from yeast,” J. Virol. 82, 5967-5980 (2008)).